Conventionally, an imaging device, such as a digital video camera and a digital still camera, that includes an optical image stabilizer (hereinafter, referred to as OIS) has been in practical use. The OIS is a mechanism for correcting burring that occurs in a captured image due to camera shake.
The OIS includes a gyro sensor that detects, in terms of an angular velocity, etc., an inclination of an imaging device which inclination is caused by camera shake. Then, the OIS corrects misalignment of an optical axis with respect to a light-receiving section of an image sensor, by shifting, in accordance with an output signal from the gyro sensor, a lens or an image sensor such as a CCD in a vertical direction with respect to the optical axis, that is, in parallel to the light-receiving section of the image sensor. This can make an object image static relative to the image sensor. As a result, blurring that occurs in a captured image due to camera shake can be corrected. This consequently makes it possible to prevent an image from blurring.
(a) of FIG. 9 is a side view illustrating an operation in image stabilization performed by a conventional camera module 200. (b) of FIG. 9 is a schematic view illustrating a captured image that is taken by the conventional camera module 200 as illustrated in (a) of FIG. 9.
As illustrated in (a) of FIG. 9, when the camera module 200 is inclined with respect to an object O due to camera shake, the camera module 200 shifts an image sensor section 105 in a negative X direction so that a center Oc of the object O is on an optical axis s. This causes a center OIc of an object image OI in a captured image to coincide with a center Ic of the captured image, as illustrated in (b) of FIG. 9. Thereby, blurring caused in the captured image by camera shake is corrected.
However, as illustrated in (b) of FIG. 9, when an image is captured under image stabilization by the conventional camera module 200, distortion or the like occurs in thus captured image because perspective (scale factor) alters in a peripheral portion of the object image OI.
FIG. 10 is a graph showing a relation between an image height and an optical distortion in a case where the camera module 200 is inclined with respect to the object O illustrated in (a) of FIG. 9. In FIG. 10, a vertical axis represents an image height (%), while a horizontal axis represents an optical distortion (%). Here, the image height indicates a distance from a center position of a captured image. In a case where the camera module 200 inclines with respect to the object O, an influence of the optical distortion on a captured image becomes more significant as the image height increases (see FIG. 10). In a case where the optical distortion is positive at a given image height, an object image captured at the given image height appears larger. Meanwhile, in a case where the optical distortion is negative at another given image height, the object image captured at this image height appears smaller. Note that the optical distortion is a ratio (value) that is obtained by dividing a difference between an ideal image height and a real image height by the ideal image height. A difference in optical distortion between maximum image heights (±100%) is a camera-movement amount. At a larger camera-movement amount, a peripheral portion of a captured image is more significantly distorted.
FIG. 11 is a graph illustrating a general image height-optical distortion curve for a lens section of the conventional camera module 200 illustrated in (a) of FIG. 9.
As illustrated in FIG. 11, in general, the lens section of the conventional camera module 200 has an image height-optical distortion curve that has characteristics such that: an optical distortion is 0 in a minimum image height (0%) region; the optical distortion is a positive maximum value in each of intermediate image height (±50%) regions; decreasing from this positive maximum value, the optical distortion becomes a negative minimum value in each of image height (±90%) regions each of which has an absolute image height percentage value that is a little lower than an absolute percentage value of each of the maximum image heights (±100%); and increasing from this minimum value, the optical distortion passes through 0 at each of the maximum image heights (±100%). In other words, the lens section of the conventional camera module 200 has an optical distortion characteristic such that the optical distortion is increasing at the maximum image heights (±100%) and in the vicinity of the maximum image heights.
FIG. 12 is a graph showing an image height-optical distortion curve of the conventional lens section in the image stabilization as illustrated in (a) of FIG. 9. As shown in FIG. 12, in the image height-optical distortion curve of the lens section in the image stabilization, an amount of change produced by a shift of the image sensor section 105 is added to an amount of change in optical distortion produced by an inclination of the camera module 200 with respect to the object O. Here, the lens section has the image height-optical distortion curve as shown in FIG. 11, that is, an optical distortion curve in which the optical distortion is increasing at the maximum image heights (±100%) and in the vicinity of the maximum image heights. In such an optical lens, the amount of change in optical distortion produced by an inclination of the camera module 200 with respect to the object O has the same polarity as the amount of change in optical distortion produced by a shift of the image sensor section 105. This increases a camera-movement amount in the image stabilization. Accordingly, the conventional camera module 200 had a problem in that a significant change occurs in perspective in a peripheral portion of an captured image OI as illustrated in (b) of FIG. 9 and such a significant change causes distortion in a captured image.
In order to solve the above problem, Patent Literature 1 proposes a technique for suppressing distortion in a captured image by digital correction with use of an ISP (Image Signal Processor). According to Patent Literature 1, the distortion in a captured image is suppressed by (i) finding out a zoom position and the like, (ii) adaptively changing a process parameter, and (iii) correcting the captured image that is stored in a memory.